There are many applications that require detecting the presence of, and if possible identifying, radioactive materials in target objects or regions. One such application is to prevent unauthorised passage of certain such materials across borders into nations or regions where such materials are prohibited. A suitable method in this border-monitoring application would be capable of performing the detection/identification as a vehicle passed through a detection zone, preferably without stopping in the zone, so as not to excessively impede the flow of traffic. Thus the method would preferably be capable of detecting the presence of prohibited materials rapidly, for example in a period of about 10 seconds or less. The method should preferably have high sensitivity, i.e. a low level of false negatives (failing to detect the presence of prohibited material) and high specificity, i.e. low false positives (signalling a detection when no prohibited material is present).
Detection of prohibited radionuclides is complicated by the fact that non-prohibited radionuclides, like prohibited radionuclides, may emit a certain level of ionising radiation, for example due to the presence of elevated concentrations of naturally occurring radioactive materials (NORMs), or of legitimate radiopharmaceutical products etc. Some existing systems, which use simple plastic scintillation detectors, measure only the gross level of radiation, in the form of gamma rays, emitted by a target. Such systems are prone to a high rate of nuisance alarms if the threshold level of radiation detection is set too low or a high rate of false negatives if the threshold level of radiation detection is set too high. Also, such systems are unable to distinguish legitimately traded goods containing elevated concentrations of NORMs from illicit or inadvertent and unlicensed goods containing prohibited radioactive materials.
A second generation of systems, known as Spectroscopic Portal Monitors (SPMs), based on Nat and HPGe detectors, seek to acquire the gamma ray spectrum of the target. Such systems contain processing to compare the acquired gamma ray spectrum with the spectra of radionuclides of interest. The spectrum processing methods have included, but not been limited to, those based on peak detection and matching, artificial neural networks, response function fitting, template matching, and wavelets.
High resolution spectroscopic equipment of the type found in SPMs is very expensive and is subject to poor reliability in field deployment due to the challenging operating conditions. Lower resolution spectroscopic equipment is less expensive and more robust but yields poorer performance with respect to radionuclide detection, namely higher rates of both false positives and false negatives.